Monoisotopic Ensembles of Silicon-Vacancy Color Centers with

Letter Cite This: ACS Photonics 2019, 6, 66−72

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Monoisotopic Ensembles of Silicon-Vacancy Color Centers with Narrow-Line Luminescence in Homoepitaxial Diamond Layers Grown in H2−CH4−[x]SiH4 Gas Mixtures (x = 28, 29, 30)

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Victor G. Ralchenko,*,†,‡,§ Vadim S. Sedov,*,† Artem K. Martyanov,† Andrey P. Bolshakov,†,‡,§ Kirill N. Boldyrev,∥ Vladimir S. Krivobok,⊥ Sergei N. Nikolaev,⊥ Stepan V. Bolshedvorskii,⊥,# Olga R. Rubinas,⊥ Alexey V. Akimov,⊥,∇,○ Andrey A. Khomich,†,◇ Egor V. Bushuev,† Roman A. Khmelnitsky,⊥ and Vitaly I. Konov†,§ †

Prokhorov General Physics Institute of Russian Academy of Sciences, 38 Vavilov str., Moscow 119991, Russia Harbin Institute of Technology, 92 Xidazhi Str., 150001 Harbin, P.R. China § National Research Nuclear University “MEPhI”, 31 Kashirskoye shosse, Moscow 115409, Russia ∥ Institute of Spectroscopy, Russian Academy of Sciences, 5 Fizicheskaya str., Troitsk, 108840 Moscow, Russia ⊥ P.N. Lebedev Physical Institute, Russian Academy of Sciences, 53 Leninsky prosp., Moscow 119991, Russia # Moscow Institute of Physics and Technology, 9 Institutskiy per., Dolgoprudny, Moscow Region 141700, Russia ∇ Russian Quantum Center, 100A Novaya str., Skolkovo, Moscow region 143025, Russia ○ Texas A & M University, 4242 TAMU, College Station, Texas 77843, United States ◇ Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Vvedensky Sq. 1, 141190 Fryazino, Russia ‡

S Supporting Information *

ABSTRACT: Silicon-vacancy (SiV−) color center in diamond is of high interest for applications in nanophotonics and quantum information technologies, as a single photon emitter with excellent spectral properties. To obtain spectrally identical SiV− emitters, we doped homoepitaxial diamond films in situ with 28Si, 29Si, and 30Si isotopes using isotopically enriched (>99.9%) silane SiH4 gas added in H2−CH4 mixtures in the course of the microwave plasma-assisted chemical vapor deposition process. Zero-phonon line components as narrow as ∼4.8 GHz were measured in both absorption and luminescence spectra for the monoisotopic SiV− ensembles with a concentration of a few parts per billion. We determined with high accuracy the Si isotopic energy shift of SiV− zero-phonon line. The SiV− emission intensity is shown to be easily controlled by the doped epifilm thickness. Also, we identified and characterized the localized single photon SiV− sources. The developed doping process opens a way to produce the SiV− emitter ensembles with energy confined in an extremely narrow range. KEYWORDS: diamond, CVD synthesis, silicon-vacancy center, doping, silicon isotope, photoluminescence, optical absorption

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a strong zero-phonon line (ZPL) at 737 nm wavelength, which contains about 70% of the fluorescence from this color center, and the weak phonon sideband.7−9 This, being coupled to the absence of spectral diffusion effects in SiV−, allows generation

olor centers in diamond, emitting across the visible and near-infrared spectrum, provide a material platform for quantum information science and applications in photonics and, more specifically, in quantum information technologies.1−4 The negatively charged silicon-vacancy (SiV−) color center in diamond is of increasing interest for nanophotonics1−7 due to its excellent spectral properties, including © 2018 American Chemical Society

Received: October 22, 2018 Published: December 10, 2018 66

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Figure 1. (a) Electronic level scheme of the SiV− center with split ground and excited states. The four ZPL transitions in PL or absorption are denoted as A−D. (b) Absorption spectra of SiV− centers with isotopes 28Si, 29Si, and 30Si. ZPL peaks shift to lower energies with Si mass increase. Dashed lines show the trends for lines A−D positions.

deposited on a single crystal CVD diamond ( H becomes less than 50%. In our confocal optical scheme, we had H ≈ 6 μm as was estimated from the geometry of collection of backscattered light and from Raman spectra measurements of CVD diamond films enriched with isotope 13C of different thicknesses.36 Therefore, the doped films with thickness up to 5 μm satisfied this criterion. To operate with SiV− single photon emitters, their area density in the doped layer preferably should be on the order of 0.2−0.4 SiV− per μm2 to be able to spatially separate the emitters (with interdistance of ∼1.5−2 μm) using a confocal optical scheme for PL photon collection.10,28 Of course, the area density of SiV− centers, up to 4 × 102 μm−2 in the films studied here, is far beyond this requirement; however, the linear plot in Figure 3 (inset) indicates a possibility to tune the total PL intensity through growth of very small thickness to form a two-dimensional ensemble of the sufficiently spaced emitters. For SiV− emitter concentration of 1.2 ppb upon our doping parameters, the film thickness adequate for SiV− density of 0.4 μm−2 should be as small as 2 nm, the value that could be achieved, in principle, using a delta-doping technique37 or by tuning the growth parameters (CH4 content and substrate temperature) in a microwave plasma to almost balance the growth and hydrogen-induced etching rates, and obtain very low growth rates.24,27 A more rational way would be, however, to reduce the Si doping level by 1 or 2 orders of magnitude by the use of highly diluted SiH4 in the process gas mixture21 to make the Si-doped layers of thickness 10−100 nm acceptable. The PL from SiV− centers in the diamond film with 28Si isotope for a sample with the reduced concentration of SiV− sites was performed at room temperature using a home-built confocal setup combined with a Hunbury Brown-Twiss interferometer38 (for details see Supporting Information). A typical distribution of the PL emission under continuous wave laser excitation (532 nm wavelength) on the area of 6.2 × 6.2 μm2 is shown in Figure 4a. The brightest emission spots are clusters of the SiV− centers, while some of less intensive sites correspond to single emitters. The autocorrelation function, g(2)(τ), taken for one such site (it is marked by the red circle in Figure 4a) displays a dip to 0.32, confirming its single photon emitter signal (Figure 4b). PL emission kinetics for a single SiV− center in this film was investigated using a picosecond diode laser (532 nm wavelength). An example of the PL intensity decay kinetics, I(t), is displayed in Figure 4c. Since the lifetime of the SiV−

Figure 4. (a) Distribution of PL emission at room temperature of SiV− centers with 28Si isotope (integrated signal within the spectral band of 740 ± 13 nm) on 6.2 × 6.2 μm2 area. (b) Autocorrelation function g(2)(τ) measured for a single SiV− center from the site shown by red circle on PL map at the laser pump power of 4.5 mW. (c) PL emission kinetics: dots with error bars are the measured data; the solid line is fitting with a single exponent function I(t) = I0 exp(−t/τ) for times t > 2 ns, with decay time τ = 1.2 ± 0.08 ns. (d) Dependence of the photoluminescence signal of a single SiV− on excitation power at 532 nm wavelength. The line is the fit I = IsP/(P + Ps).

center excited state, on the order of 1 ns, is much longer than the length of the laser pulse, the PL signal recorded after the end of the laser pulse exhibits exponential decay; the actual signal observed consist of two exponents, a fast and a slow one. The fast exponential component corresponds to the background fluorescence signatures that do not have a slowly decaying exponential component. This was verified by focusing the microscope on a region containing no optically active color centers in the diamond film (dark areas in Figure 4a) and measuring the background fluorescence decay time collected from that region. The decay curve for times t > 2 ns is well fitted with an exponential function, I(t) = I0 exp(−t/τ). We, therefore, associate the long-lived exponential tail decay time τ = 1.2 ± 0.08 ns in Figure 4c with PL of the SiV− center. This value is in agreement with literature data on SiV− PL kinetics for different diamond materials and Si-doping methods, including CVD diamond nanopillars doping from silane (τ = 1.0 ns),39 ion implantation (1.2 ns) in type IIa natural diamond films,40 doping of isolated diamond nanoparticles (1.1 ns)9 and homoepitaxial diamond films (1.3 ns)41 with silicon in course of CVD process. To assess the maximum possible count rate, we measured the power dependence of the count rate for the selected single SiV− center (Figure 4d). As the absorption maximum of the SiV− center is in the near IR (738 nm), it is relatively difficult to reach experimentally complete saturation with green excitation.42 We fitted the obtained dependence using the P standard saturation model,9 Is P + P , where I is the measured s

count rate, P is excitation power, and fitting parameters are the saturation power, Ps, and the extrapolated maximum number of counts, Is. The background fluorescence has been subtracted. Following this procedure, it was found, that the SiV− center in the diamond film has the fluorescence count rate Is = 89 ± 11 kilocounts/s and saturation power Ps = 8.6 mW. This is quite comparable with single SiV− obtained by ion implantation followed by annealing under green excitation at room

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temperature.44 Nevertheless, the difference in count rates significantly depends on collection efficiency in the confocal setup. We estimated the collection efficiency of 0.2% for our experiment (see Supporting Information for details). In summary, we demonstrated the controllable in situ doping of epitaxial diamond layers with isotopically enriched silicon (28Si, 29Si, and 30Si) by adding silane SiH4 gas in H2−CH4 mixture in the course of the microwave plasma-assisted CVD process to form ensembles of monoisotopic SiV− color centers with very narrow (∼4.8 GHz) ZPL components. The presence of the SiV− centers at 737 nm wavelength was revealed both in photoluminescence and in optical absorption spectra for each of the Si isotopes. The zero-phonon line in SiV− absorption at low temperature (5 K) exhibits a spectral shift to lower energies with Si mass increase, namely, by 0.69 ± 0.01 meV when the 28Si isotope is replaced by the 30Si, with a similar trend for ZPL in PL spectra. Based on the spectral shift of the peak around 64 meV in the phonon band, we confirmed it to be a local vibration mode related to a single Si atom, in agreement with the previously reported finding by Dietrich et al.15 for measurements of individual SPE within SiV− ensemble with natural Si isotope composition. The SiV− PL intensity is shown to linearly depend on the doped film thickness making it possible to control the area density of the color centers, in particular, by depositing ultrathin epitaxial films with rarely arranged centers. The developed doping method can be a viable alternative to Si ion implantation in diamond12,45,46 to form isotopically pure SiV− centers as identical single photon emitters. Our results have an impact on the application of single-photon sources for quantum optical information technologies. The doping with Si using CVD method potentially promises to form SiV− centers in very shallow layers using a special design of CVD system with a rapid change of gas flow, as was demonstrated recently for doping with boron and nitrogen.34,37 With such strong depth localization, the CVD doping could compete with ion implantation techniques.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to E. E. Ashkinazi for preparation of diamond substrates and to G. G. Devyatykh Institute of Chemistry of High-Purity Substances of Russian Academy of Sciences, Nizhny Novgorod, Russia, for the production of isotopically pure [x]SiH4 gases. This work was supported by the Russian Science Foundation, Grant No. 14-12-01403-P.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01464. Details of diamond growth and doping procedures; annealing effect; spectroscopic setup, measuring the absolute concentration and mapping of SiV− centers (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Vadim S. Sedov: 0000-0001-9802-1106 Artem K. Martyanov: 0000-0002-3196-5695 Alexey V. Akimov: 0000-0002-4167-5085 Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. 71

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